Mass separation of high energy particles



Sept. 1962 L. MARSHALL 3,056,023

MASS SEPARATION OF HIGH ENERGY PARTICLES Filed Nov. 18, 1960 INVENTOR.

LEONA MARSHALL BY flM W i tats This invention relates to a method and apparatus for accomplishing the mass separation of high energy particles and more particularly to a method and apparatus for separating out high energy particles of equal momentum but differing in masses.

In high energy particles physics, it is often desirable to separate out particles of equal momentum but differing in mass having energies on the order of several billion electron volts. Various proposals have been made and used to accomplish such separation, including those arrangements shown in applications Ser. Nos. 16,902, filed March 22, 1960, now Patent No. 3,016,458, issued January 9, 1962, and 41,708, filed July 8, 1960. While these arrangements are useful and can accomplish the desired separation, they do require the manufacture and assembly of wave guide devices in which accuracy of dimensions and suitability for the exact frequencies of operation are necessary. Furthermore, once the wave guides are constructed for their particular use, it is difiicult, if not impossible, to modify and make them suitable for other frequencies of operation.

By the present invention, there is provided method and apparatus for the separation of high energy particles produced by accelerating devices operating in the multiple bev. range, such as the Alternating Gradient Synchrotron at the Brookhaven National Laboratory. The method and construction involved in this invention utilizes standard and well known techniques and apparatus and is highly flexible for a wide range of applications, energies, frequencies, and types of particles. More specifically, the invention involves the use of systems of magnetic lenses which are used to focus the beams of particles to be separated. The systems of magnetic lenses are arranged, in one embodiment, to cooperate with electrostatic fields imposed between successive magnetic lenses to build up over the length of the apparatus increasing transverse separation of the different types of particles. With the proper selection of magnetic lenses and electrostatic fields and a sufiicient number of focusing stages, as hereinafter to be more particularly described, it is possible by this invention to obtain the desired separation.

It is thus a first object of this invention to provide a method and apparatus for the mass separation of high energy particles.

It is a further object of this invention to provide a quadrupole lens focusing system in which the mass separation of high energy particles takes place.

It is still another object of this invention to focus a beam of high energy particles having identical momenta and to separate out the particles of different mass.

Still another object of this invention is provision for handling high energy nuclear particles of equal momentum and obtaining their mass separation.

The exact nature of this invention as Well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the accompanying drawing in which:

FIG. 1 is a diagrammatic illustration of the principles involved in this invention; and

FIG. 2 is a simple magnetic lens system in which a small degree of separation of particles is obtained.

It is known that beams of high energy particles may be piped over long distances by using systems of quadrupole magnetic lenses. A quadrupole magnetic lens is shown and discussed in The Strong-Focusing Synchrotron-A New High Energy Accelertator, by E. D. Courant, M. S. Livingnston, and H. S. Snyder, in The Physical Review, vol. 88, No. 5, pages 1190-1196, December 1, 1952. FIGURE 9 of that paper illustrates a lens of this type. Focusing of the particles by this type of lens system is based on the fact that all the particles making up a particular beam have identical momentum, and the particles and mathematics of such focusing systems are well kn wn in the art. The invention hereinafter described utilizes a focusing system of this type in which the spaces between the lenses through which the particles pass are subjected to electric fields in a manner described below in connection with particular configurations of these magnetic focusing systems. A mathematical treatment of the principles involved in this invention is given in my article coauthored with E. D. Courant on Mass Separation of High Energy Particles and Quadrupole Lens Focusing Systems, published in The Review of Scientific Instruments, volume 31, No. 2, pages 193-196, February 1960.

In order to explain how the method and apparatus of this invention accomplishthe function of separating the particles of diflferent mass and the same momentum, reference is made to FIGURE 1 wherein is illustrated an axis Z-Z representing the path prior to alteration traveled by a beam of particles to undergo separation. Disposed along the length of axis Z-Z are a plurality of quadrupole lens pairs M1, M2and M3, each quadrupole of each pair, as is understood in the art, tending to focus the particles passing therethrough in some particular transverse plane, such as either the X or Y planes, at right angles to each other and passing through the axis Z-Z, as is understood in the art. Each lens pair consists of two quadrupole magnetic lenses to obtain complete focusing in all planes. Consider a path 12 taken by a charged particle 13 as a result of the influence of lens pairs M1, M2 and M3, which are oriented to exert forces on particle 13 in the following manner. It will be seen that particle 13 following path 12 intersects the ZZ axis at places designated converging points P1, P2 and P3. Particles similarly charged but having different directions of momentum follow paths 16, 18, and 14, for example. Since the total momentum of the various particles are identical, they all pass through the identical points P1, P2 and P3.

Should electrostatic fields be applied to particle 13 in between lenses M1, M2 and M3 in the directions indicated by arrows A and B, it is seen that the converging points P2 and P3 will be translated accordingly. For example, point P2 will become P2 for particle 13, and point P3 Will become P3. The particles of opposite polarity will, of course, be acted upon in an opposite direction by the same electrostatic field and their node position (not shown) would be on the other side of focal points P2 and P3. Also the displacement of the point P3 from axis Z-Z will be substantially twice that of the displacement of point P2, for the effects of the electrostatic fields would be cumulative. If no electrostatic field were applied to the area between lenses M2 and M3 the new point P3 would be the same distance below P3 as P2 is above P2. However, with the application of this new electrostatic field, to the particles passing through point P3, the additional force causes an increase in the displacement transverse to axis ZZ as mentioned. Over a plurality of such stages, the spread of the various particles will be distributed in a transverse plane off axis ZZ in accordance with their masses. This follows because the transverse acceleration of a particle by the magnetic field force varies inversely as the momentum whereas that due to electric field varies inversely as momentum times velocity. Particles of given momentum but different masses have different velocities. With proper apparatus and appropriately sized apertures, it becomes possible, if desired, to pick olf particles of a specific mass.

A practical arrangement for accomplishing the mass separation of particles having identical momentum and charge is illustrated in FIGURE 2. Shown there is an arrangement of magnetic lenses L1, L2, L3 and L4 and a source of particles to undergo separation. In order to accomplish the desired separation, source 0 is located at point f1 on the central aixs of the lens system and separated a distance from lens L1 so that all of the particles entering lens L1 from source 0 will leave in the form of a parallel beam 19. Thus, the focal length for lens L1, making the usual analogy to optical lenses, is the distance between source 0 and lens L1, hence source 0 is located at point f1. Lens L2 receives the parallel beam 19 from lens L1 after a distance of S. The rays upon leaving lens L2 will converge at a point f2 which may be described as the focal point for lens L2. The beam of particles crosses point f2 and enters L3 which is located from point f2 a distance equal to its focal length. Thus, the particles emerge from lens L3 in a parallel beam and after a suitable distance S the beam of particles enters lens L4. The beam of particles converge at the focal point f3 and continue on in the manner previously described. In the arrangement illustrated, lenses L1, L2, L3 and L4 are each a single quadrupole so that FIG. 2 illustrates one half the arrangement required for the first focusing point. At least two focusing nodes for each transverse plane are required in order to obtain sufficient separation of the beams, and the electrostatic fields must be reversed to take advantage of the resonant effect described in connection with FIG. 1.

To obtain the desired separation of the particles in accordance with the principles desired in connection with FIG. 1, four pairs of electrostatic plates 20, 22; 24, 26; 28, 30; and 3.2, 34 are mounted between adjacent lenses in the manner illustrated and as generally understood in the art. The first two sets of electrostatic plates -26 are biased to exert a force on the particles emerging from source 0 in a direction transverse to the axis of the lens as indicated by arrows C and D, for a particular charge. The second two sets of electrostatic plates 28 through 34 exert forces on the particles in the opposite direction along the direction illustrated by arrows E and F. This is due to the crossing of the beam of particles at focal point f2 and is designed to make the separation cumulative as previously described. Thus focal point f2 is translated to a point above focal point f2 and focal point f3 is moved to a focal point below f3. If desired, the electrostatic fields may be reversed at the focal points f2 and f3.

It has already been noted that unless lenses L1, L2, etc. are lens pairs, focusing occurs in one plane, and defocusing occurs in the other transverse plane. Thus a minimum complete arrangement of the type shown in FIGURE 2. would be, as already noted, a repetition of the arrangement shown from source 0 to focal point f3 with focusing occurring in an appropriate way in the previously defocused plane. With lenses L1, L2, etc. being lens pairs, then a complete arrangement extends from source 0 to point f3. The distances S and S are limited by the size of the lenses and in accordance with the considerations mentioned in The Review of Scientific Instruments paper. To increase separation of the particles, additional lens arrangements of the type just described would be added on, except that the separation to be useful must be greater than the product of the over-all magnification of the lens system and the source size 0.

It is possible to replace the lens stages required by substitution using lenses known as triplets which focus simultaneously in the X and Y transverse planes. A triplet magnetic lens, as is understood in the art, consists of three successive magnet quadrupole arrangements in succession combined into a single unit in which the first and third lenses are opposite in polarity to the middle, or second one, and half the strength of the central one.

It is thus seen that there has been provided a novel method and apparatus for accomplishing the mass separation of high energy particles utilizing quadrupole lens focusing systems. It is understood, of course, that only a preferred embodiment has been illustrated and many variations of that shown may be provided utilizing these principles.

It is also understood that suitable piping apparatus under vacuum for the handling of these particles will increase the efficiency of the method and apparatus herein described and that a suflicient number of stages may be added to accomplish the desired separation. It is also understood that the electrostatic fields, while not described particularly, may be either constant or pulsating depending upon the type of source utilized which may be pulsating. In fact, the magnets, as well as the sources of the electrostatic fields, may be pulses to obtain the desired results.

As the foregoing relates to a preferred embodiment of this invention and numerous modifications and alternating thereof may be made therein without departing from the spirit and the scope of the invention, it is intended that the appended claims define the scope of this invention.

I claim:

1. Particle separating apparatus for use with a source of highly energized charged particles of identical mo mentum and differing in mass, comprising, means consisting of a system of magnetic lenses for receiving and focusing said particles in successive stages, and means for applying electrostatic fields to said particles in said lens system in the spaces between said lenses for effecting the distribution of said particles along transverse planes in accordance with their masses for facilitating their separation.

2. The particle separating apparatus of claim 1 in which said magnetic lenses consist of quadrupole magnets used in pairs rotated from each other to focus said beam of particles.

3. The particle separating apparatus of claim 1 in which said lens system includes two focusing nodes for each set of lenses focusing in a transverse plane to prevent increase in beam width which would limit amount of separation possible.

4. Particle separating apparatus for use with a source of highly energized charged particles having equal momentum and differing in mass, comprising a plurality of magnetic lenses for receiving and focusing particles in successive stages, said source being located at the focal point of the first of said magnetic lenses, the second of said magnetic lenses separated from said first lens to receive the particles forming a parallel beam from said first lens and focus said particles at the focal point of said second lens, a third magnetic lens located with its focal point coinciding with the focal point of said second lens to receive said beam of particles, and means imposing electrostatic fields in a direction perpendicular to the axis of said lens system on the particle between said first and second, and second and third lenses, respectively, to effect the displacement of said particles transverse to their beam travel in accordance with their masses.

5. In a method for separating highly energized 5 charged particles of identical momentum and differing ration of N times that of a single section where N secmasses forming a beam successively focused and defotions are employed, cused in a system of quadrupole magnetic lenses, the step of imposing an electrostatic field on said particles to References Cited in the file of this patent bend said particles away from the axis of said lens sys- 5 tern in accordance with the masses of said particles. UNITED STATES PATENTS 6. The method of claim 5 in which the polarity of the electrostatic field is alternated in successive sections 2886727 Hame May 1959 of said system of magnetic lenses to obtain a total sepa- 2,919,381 Glaser 29, 1959 

